Empiricism, in philosophy, the view that all concepts originate in experience, that all concepts are about or applicable to things that can be experienced, or that all rationally acceptable beliefs or propositions are justifiable or knowable only through experience. This broad definition accords with the derivation of the term empiricism from the ancient Greek word empeiria, “experience.”

Concepts are said to be “a posteriori” (Latin: “from the latter”) if they can be applied only on the basis of experience, and they are called “a priori” (“from the former”) if they can be applied independently of experience. Beliefs or propositions are said to be a posteriori if they are knowable only on the basis of experience and a priori if they are knowable independently of experience (seea posteriori knowledge). Thus, according to the second and third definitions of empiricism above, empiricism is the view that all concepts, or all rationally acceptable beliefs or propositions, are a posteriori rather than a priori.

The first two definitions of empiricism typically involve an implicit theory of meaning, according to which words are meaningful only insofar as they convey concepts. Some empiricists have held that all concepts are either mental “copies” of items that are directly experienced or complex combinations of concepts that are themselves copies of items that are directly experienced. This view is closely linked to the notion that the conditions of application of a concept must always be specified in experiential terms.

The third definition of empiricism is a theory of knowledge, or theory of justification. It views beliefs, or at least some vital classes of belief—e.g., the belief that this object is red—as depending ultimately and necessarily on experience for their justification. An equivalent way of stating this thesis is to say that all human knowledge is derived from experience.

Empiricism regarding concepts and empiricism regarding knowledge do not strictly imply each other. Many empiricists have admitted that there are a priori propositions but have denied that there are a priori concepts. It is rare, however, to find a philosopher who accepts a priori concepts but denies a priori propositions.

Stressing experience, empiricism often opposes the claims of authority, intuition, imaginative conjecture, and abstract, theoretical, or systematic reasoning as sources of reliable belief. Its most fundamental antithesis is with the latter—i.e., with rationalism, also called intellectualism or apriorism. A rationalist theory of concepts asserts that some concepts are a priori and that these concepts are innate, or part of the original structure or constitution of the mind. A rationalist theory of knowledge, on the other hand, holds that some rationally acceptable propositions—perhaps including “every thing must have a sufficient reason for its existence” (the principle of sufficient reason)—are a priori. A priori propositions, according to rationalists, can arise from intellectual intuition, from the direct apprehension of self-evident truths, or from purely deductive reasoning.

Various meanings of empiricism

Broader senses

In both everyday attitudes and philosophical theories, the experiences referred to by empiricists are principally those arising from the stimulation of the sense organs—i.e., from visual, auditory, tactile, olfactory, and gustatory sensation. (In addition to these five kinds of sensation, some empiricists also recognize kinesthetic sensation, or the sensation of movement.) Most philosophical empiricists, however, have maintained that sensation is not the only provider of experience, admitting as empirical the awareness of mental states in introspection or reflection (such as the awareness that one is in pain or that one is frightened); such mental states are then often described metaphorically as being present to an “inner sense.” It is a controversial question whether still further types of experience, such as moral, aesthetic, or religious experience, ought to be acknowledged as empirical. A crucial consideration is that, as the scope of “experience” is broadened, it becomes increasingly difficult to distinguish a domain of genuinely a priori propositions. If, for example, one were to take the mathematician’s intuition of relationships between numbers as a kind of experience, one would be hard-pressed to identify any kind of knowledge that is not ultimately empirical.

Even when empiricists agree on what should count as experience, however, they may still disagree fundamentally about how experience itself should be understood. Some empiricists, for example, conceive of sensation in such a way that what one is aware of in sensation is always a mind-dependent entity (sometimes referred to as a “sense datum”). Others embrace some version of “direct realism,” according to which one can directly perceive or be aware of physical objects or physical properties (seeepistemology: realism). Thus there may be radical theoretical differences even among empiricists who are committed to the notion that all concepts are constructed out of elements given in sensation.

Two other viewpoints related to but not the same as empiricism are the pragmatism of the American philosopher and psychologist William James, an aspect of which was what he called radical empiricism, and logical positivism, sometimes also called logical empiricism. Although these philosophies are empirical in some sense, each has a distinctive focus that warrants its treatment as a separate movement. Pragmatism stresses the involvement of ideas in practical experience and action, whereas logical positivism is more concerned with the justification of scientific knowledge.

When describing an everyday attitude, the word empiricism sometimes conveys an unfavourable implication of ignorance of or indifference to relevant theory. Thus, to call a doctor an “Empiric” has been to call him a quack—a usage traceable to a sect of medical men who were opposed to the elaborate medical—and in some views metaphysical—theories inherited from the Greek physician Galen of Pergamum (129–c. 216 ce). The medical empiricists opposed to Galen preferred to rely on treatments of observed clinical effectiveness, without inquiring into the mechanisms sought by therapeutic theory. But empiricism, detached from this medical association, may also be used, more favourably, to describe a hard-headed refusal to be swayed by anything but the facts that the thinker has observed for himself, a blunt resistance to received opinion or precarious chains of abstract reasoning.

Stricter senses

As a more strictly defined movement, empiricism reflects certain fundamental distinctions and occurs in varying degrees.

A distinction that has the potential to create confusion is the one that contrasts the a posteriori not with the a priori but with the innate. Since logical problems are easily confused with psychological problems, it is difficult to disentangle the question of the causal origin of concepts and beliefs from the question of their content and justification.

A concept, such as “five,” is said to be innate if a person’s possession of it is causally independent of his experience—e.g., his perception of various groupings of five objects. Similarly, a belief is innate if its acceptance is causally independent of the believer’s experience. It is therefore possible for beliefs to be innate without being a priori: for example, the baby’s belief that its mother’s breast will nourish it is arguably causally independent of his experience, though experience would be necessary to justify it.

Another supposedly identical, but in fact more or less irrelevant, property of concepts and beliefs is that of the universality of their possession or acceptance—that a priori or innate concepts and beliefs must be held by everyone. There may be, in fact, some basis for inferring universality from innateness, since many innate characteristics, such as the fear of loud noises, appear to be common to the whole human species. But there is no inconsistency in the supposition that a concept or belief is innate in one person and learned from experience in another.

Two main kinds of concept have been held to be a priori. First, there are certain formal concepts of logic and of mathematics that reflect the basic structure of discourse: “not,” “and,” “or,” “if,” “all,” “some,” “existence,” “unity,” “number,” “successor,” and “infinity.” Secondly, there are the “categorial” concepts—such as “substance,” “cause,” “mind,” and “God”—which, according to some philosophers, are imposed by the mind upon the raw data of sensation in order to make experiences possible. One might add to these the more specific theoretical concepts of physics, which are sometimes said to apply to entities that are unobservable in principle.

In the long history of debate over the a priori, it was long taken for granted that all a priori propositions are necessarily true—i.e., true by virtue of the meanings of their terms (“analytic”) or true by virtue of the fact that their negations imply a contradiction. Propositions such as “all triangles have three sides,” “all bachelors are unmarried,” and “all red things are coloured” are necessarily true in one or both of these senses. Likewise, it was held that propositions that are contingently true, or true merely by virtue of the way the world happens to be, are a posteriori. “John is a bachelor” and “John’s house is red” are propositions of this type.

In the 1970s, however, the American philosopher Saul Kripke argued to the contrary that some a priori propositions are contingent and some a posteriori propositions are necessary. According to Kripke, the referential properties of “natural kind” terms like heat can be understood by imagining that their referents were fixed, upon their introduction into the language, by means of certain definite descriptions, such as “the cause of sensations of warmth.” In other words, heat was introduced as a name for whatever phenomenon happened to satisfy the description “the cause of sensations of warmth.” Of course, the phenomenon in question is now known to be molecular motion. Thus heat refers to molecular motion, then and now, because molecular motion was the cause of sensations of warmth when the term was introduced. Given this introduction, however, the proposition “heat causes sensations of warmth” must be a priori. Because its introduction stipulated that heat is the phenomenon that causes sensations of warmth, it is knowable independently of experience that heat causes sensations of warmth, even though it is only a contingent matter of fact that it does. On the other hand, the proposition “heat is molecular motion” is a posteriori, because this fact about heat was discovered (and could only be discovered) through empirical scientific investigation. But the proposition is also necessary, according to Kripke, because once the referent of heat has been fixed as molecular motion, there are no imaginable circumstances in which the term could refer to anything else. This conclusion is supported by the intuition that, if it were discovered tomorrow that sensations of warmth in humans are actually caused by something other than molecular motion, one would not say that heat is not molecular motion but rather that sensations of warmth are caused by something other than heat. Kripke proposed a similar analysis of the referential properties of proper names like “Aristotle,” according to which a proposition like “Aristotle was the teacher of Alexander the Great” is contingent but a priori.

Empiricism, whether concerned with concepts or knowledge, can be held with varying degrees of strength. On this basis, absolute, substantive, and partial empiricisms can be distinguished.

Absolute empiricism

Absolute empiricists hold that there are no a priori concepts, either formal or categorial, and no a priori beliefs or propositions. Absolute empiricism about the former is more common than that about the latter, however. Although nearly all Western philosophers admit that obvious tautologies (e.g., “all red things are red”) and definitional truisms (e.g., “all triangles have three sides”) are a priori, many of them would add that these represent a degenerate case.

Substantive empiricism

A more moderate form of empiricism is that of the substantive empiricists, who are unconvinced by attempts that have been made to interpret formal concepts empirically and who therefore concede that formal concepts are a priori, though they deny that status to categorial concepts and to the theoretical concepts of physics, which they hold are a posteriori. According to this view, allegedly a priori categorial and theoretical concepts are either defective, reducible to empirical concepts, or merely useful “fictions” for the prediction and organization of experience.

The parallel point of view about knowledge assumes that the truth of logical and mathematical propositions is determined, as is that of definitional truisms, by the relationships between meanings that are established prior to experience. The truth often espoused by ethicists, for example, that one is truly obliged to rescue a person from drowning only if it is possible to do so, is a matter of meanings and not of facts about the world. On this view, all propositions that, in contrast to the foregoing example, are in any way substantially informative about the world are a posteriori. Even if there are a priori propositions, they are formal or verbal or conceptual in nature, and their necessary truth derives simply from the meanings that attached to the words they contain. A priori knowledge is useful because it makes explicit the hidden implications of substantive, factual assertions. But a priori propositions do not themselves express genuinely new knowledge about the world; they are factually empty. Thus “All bachelors are unmarried” merely gives explicit recognition to the commitment to describe as unmarried anyone who has been described as a bachelor.

Substantive empiricism about knowledge regards all a priori propositions as being more-or-less concealed tautologies. If a person’s “duty” is thus defined as that which he should always do, the statement “A person should always do his duty” then becomes “A person should always do what he should always do.” Deductive reasoning is conceived accordingly as a way of bringing this concealed tautological status to light. That such extrication is nearly always required means that a priori knowledge is far from trivial.

For the substantive empiricist, truisms and the propositions of logic and mathematics exhaust the domain of the a priori. Science, on the other hand—from the fundamental assumptions about the structure of the universe to the singular items of evidence used to confirm its theories—is regarded as a posteriori throughout. The propositions of ethics and those of metaphysics, which deals with the ultimate nature and constitution of reality (e.g., “only that which is not subject to change is real”), are either disguised tautologies or “pseudo-propositions”—i.e., combinations of words that, despite their grammatical respectability, cannot be taken as true or false assertions at all.

Partial empiricism

The least thoroughgoing type of empiricism here distinguished, ranking third in degree, can be termed partial empiricism. According to this view, the realm of the a priori includes some concepts that are not formal and some propositions that are substantially informative about the world. The theses of the transcendental idealism of Immanuel Kant (1720–1804), the general scientific conservation laws, the basic principles of morality and theology, and the causal laws of nature have all been held by partial empiricists to be both “synthetic” (substantially informative) and a priori. As noted above, philosophers who embrace the Kripkean notion of reference fixing would add to this class propositions such as “heat is the cause of sensations of warmth” and “Aristotle was the teacher of Alexander the Great,” both of which derive their presumed aprioricity from the hypothetical circumstances in which their subject terms were introduced. At any rate, in all versions of partial empiricism there remain a great many straightforwardly a posteriori concepts and propositions: ordinary singular propositions about matters of fact and the concepts that figure in them are held to fall in this domain.

History of empiricism

Ancient philosophy

So-called common sense might appear to be inarticulately empiricist; and empiricism might be usefully thought of as a critical force resisting the pretensions of a more speculative rationalist philosophy. In the ancient world the kind of rationalism that many empiricists oppose was developed by Plato (c. 428–c. 328 bce), the greatest of rationalist philosophers. The ground was prepared for him by three earlier bodies of thought: the Ionian cosmologies of the 6th century bce, with their distinction between sensible appearance and a reality accessible only to pure reason; the philosophy of Parmenides (early 5th century bce), the important early monist, in which purely rational argument is used to prove that the world is really an unchanging unity; and Pythagoreanism, which, holding that the world is really made of numbers, took mathematics to be the repository of ultimate truth.

Plato (left) and Aristotle, detail from School of Athens, fresco by …

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The first empiricists in Western philosophy were the Sophists, who rejected such rationalist speculation about the world as a whole and took humanity and society to be the proper objects of philosophical inquiry. Invokingskeptical arguments to undermine the claims of pure reason, they posed a challenge that invited the reaction that comprised Plato’s philosophy.

Plato, and to a lesser extent Aristotle, were both rationalists. But Aristotle’s successors in the ancient Greek schools of Stoicism and Epicureanism advanced an explicitly empiricist account of the formation of human concepts. For the Stoics the human mind is at birth a clean slate, which comes to be stocked with concepts by the sensory impingement of the material world upon it. Yet they also held that there are some concepts or beliefs, the “common notions,” that are present to the minds of all humans; and these soon came to be conceived in a nonempirical way. The empiricism of the Epicureans, however, was more pronounced and consistent. For them human concepts are memory images, the mental residues of previous sense experience, and knowledge is as empirical as the ideas of which it is composed.

Medieval philosophy

Most medieval philosophers after St. Augustine (354–430) took an empiricist position, at least about concepts, even if they recognized much substantial but nonempirical knowledge. The standard formulation of this age was: “There is nothing in the intellect that was not previously in the senses.” Thus St. Thomas Aquinas (1225–74) rejected innate ideas altogether. Both soul and body participate in perception, and all ideas are abstracted by the intellect from what is given to the senses. Human ideas of unseen things, such as angels and demons and even God, are derived by analogy from the seen.

The 13th-century scientist Roger Bacon emphasized empirical knowledge of the natural world and anticipated the polymath Renaissancephilosopher of scienceFrancis Bacon (1561–1626) in preferring observation to deductive reasoning as a source of knowledge. The empiricism of the 14th-century Franciscan nominalistWilliam of Ockham was more systematic. All knowledge of what exists in nature, he held, comes from the senses, though there is, to be sure, “abstractive knowledge” of necessary truths; but this is merely hypothetical and does not imply the existence of anything. His more extreme followers extended his line of reasoning toward a radical empiricism, in which causation is not a rationally intelligible connection between events but merely an observed regularity in their occurrence.

Modern philosophy

In the earlier and unsystematically speculative phases of Renaissance philosophy, the claims of Aristotelian logic to yield substantial knowledge were attacked by several 16th-century logicians; in the same century, the role of observation was also stressed. One mildly skeptical Christian thinker, Pierre Gassendi (1592–1655), advanced a deliberate revival of the empirical doctrines of Epicurus. But the most important defender of empiricism was Francis Bacon, who, though he did not deny the existence of a priori knowledge, claimed that, in effect, the only knowledge that is worth having (as contributing to the relief of the human condition) is empirically based knowledge of the natural world, which should be pursued by the systematic—indeed almost mechanical—arrangement of the findings of observation and is best undertaken in the cooperative and impersonal style of modern scientific research. Bacon was, in fact, the first to formulate the principles of scientific induction.

A materialist and nominalist, Thomas Hobbes (1588–1679) combined an extreme empiricism about concepts, which he saw as the outcome of material impacts on the bodily senses, with an extreme rationalism about knowledge, of which, like Plato, he took geometry to be the paradigm. For him all genuine knowledge is a priori, a matter of rigorous deduction from definitions. The senses provide ideas; but all knowledge comes from “reckoning,” from deductive calculations carried out on the names that the thinker has assigned to them. Yet all knowledge also concerns material and sensible existences, since everything that exists is a body. (On the other hand, many of the most important claims of Hobbes’s ethics and political philosophy certainly seem to be a posteriori, insofar as they rely heavily on his experience of human beings and the ways in which they interact.)

The most elaborate and influential presentation of empiricism was made by John Locke (1632–1704), an early Enlightenment philosopher, in the first two books of his Essay Concerning Human Understanding (1690). All knowledge, he held, comes from sensation or from reflection, by which he meant the introspective awareness of the workings of one’s own mind. Locke often seemed not to separate clearly the two issues of the nature of concepts and the justification of beliefs. His Book I, though titled “Innate Ideas,” is largely devoted to refuting innate knowledge. Even so, he later admitted that much substantial knowledge—in particular, that of mathematics and morality—is a priori. He argued that infants know nothing; that if humans are said to know innately what they are capable of coming to know, then all knowledge is, trivially, innate; and that no beliefs whatever are universally accepted. Locke was more consistent about the empirical character of all concepts, and he described in detail the ways in which simple ideas can be combined to form complex ideas of what has not in fact been experienced. One group of dubiously empirical concepts—those of unity, existence, and number—he took to be derived both from sensation and from reflection. But he allowed one a priori concept—that of substance—which the mind adds, seemingly from its own resources, to its conception of any regularly associated group of perceptible qualities.

John Locke, oil on canvas by Herman Verelst, 1689; in the National Portrait Gallery, London.

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Bishop George Berkeley (1685–1753), a theistic idealist and opponent of materialism, applied Locke’s empiricism about concepts to refute Locke’s account of human knowledge of the external world. Because Berkeley was convinced that in sense experience one is never aware of anything but what he called “ideas” (mind-dependent qualities), he drew and embraced the inevitable conclusion that physical objects are simply collections of perceived ideas, a position that ultimately leads to phenomenalism—i.e., to the view that propositions about physical reality are reducible to propositions about actual and possible sensations. He accounted for the continuity and orderliness of the world by supposing that its reality is upheld in the perceptions of an unsleeping God. The theory of spiritual substance involved in Berkeley’s position seems to be vulnerable, however, to most of the same objections as those that he posed against Locke. Although Berkeley admitted that he did not have an idea of mind (either his own or the mind of God), he claimed that he was able to form what he called a “notion” of it. It is not clear how to reconcile the existence of such notions with a thoroughgoing empiricism about concepts.

The Scottish skeptical philosopher David Hume (1711–76) fully elaborated Locke’s empiricism and used it reductively to argue that there can be no more to the concepts of body, mind, and causal connection than what occurs in the experiences from which they arise. Like Berkeley, Hume was convinced that perceptions involve no constituents that can exist independently of the perceptions themselves. Unlike Berkeley, he could find neither an idea nor a notion of mind or self, and as a result his radical empiricism contained an even more parsimonious view of what exists. While Berkeley thought that only minds and their ideas exist, Hume thought that only perceptions exist and that it is impossible to form an idea of anything that is not a perception or a complex of perceptions. For Hume all necessary truth is formal or conceptual, determined by the various relations that hold between ideas.

Voltaire (1694–1778) imported Locke’s philosophy into France. Its empiricism, in a very stark form, became the basis of sensationalism, in which all of the constituents of human mental life are analyzed in terms of sensations alone.

A genuinely original and clarifying attempt to resolve the controversy between empiricists and their opponents was made in the transcendental idealism of Kant, who drew upon both Hume and Gottfried Wilhelm Leibniz (1646–1716). With the dictum that, although all knowledge begins with experience it does not all arise from experience, he established a clear distinction between the innate and the a priori. He held that there are a priori concepts, or categories—substance and cause being the most important—and also substantial or synthetic a priori truths. Although not derived from experience, the latter apply to experience. A priori concepts and propositions do not relate to a reality that transcends experience; they reflect, instead, the mind’s way of organizing the amorphous mass of sense impressions that flow in upon it.

Lockean empiricism prevailed in 19th-century England until the rise of Hegelianism in the last quarter of the century (see alsoGeorg Wilhelm Friedrich Hegel). To be sure, the Scottish philosophers who followed Hume but avoided his skeptical conclusions insisted that humans do have substantial a priori knowledge. But the philosophy of John Stuart Mill (1806–73) is thoroughly empiricist. He held that all knowledge worth having, including mathematics, is empirical. The apparent necessity and aprioricity of mathematics, according to Mill, is the result of the unique massiveness of its empirical confirmation. All real knowledge for Mill is inductive and empirical, and deduction is sterile. (It is not clear that Mill consistently adhered to this position, however. In both his epistemology and his ethics, he sometimes seemed to recognize the need for first principles that could be known without proof.) The philosopher of evolutionHerbert Spencer (1820–1903) offered another explanation of the apparent necessity of some beliefs: they are the well-attested (or naturally selected) empirical beliefs inherited by living humans from their evolutionary ancestors. Two important mathematicians and pioneers in the philosophy of modern physics, William Kingdon Clifford (1845–79) and Karl Pearson (1857–1936), defended radically empiricist philosophies of science, anticipating the logical empiricism of the 20th century.

Contemporary philosophy

The most influential empiricist of the 20th century was the great British philosopher and logician Bertrand Russell (1872–1970). Early in his career Russell admitted both synthetic a priori knowledge and concepts of unobservable entities. Later, through discussions with his pupil Ludwig Wittgenstein (1889–1951), Russell became convinced that the truths of logic and mathematics are analytic and that logical analysis is the essence of philosophy. In his empiricist phase, Russell analyzed concepts in terms of what one is “directly acquainted” with in experience (where experience was construed broadly enough to include not only awareness of sense data but also awareness of properties construed as universals). In his neutral monist phase, he tried to show that even the concepts of formal logic are ultimately empirical, though the experience that supplies them may be introspective instead of sensory.

Bertrand Russell, c. 1951.

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Doctrines developed by Russell and Wittgenstein influenced the German-American philosopher Rudolf Carnap (1891–1970) and the Vienna Circle, a discussion group in which the philosophy of logical positivism was developed. The empirical character of logical positivism is especially evident in its formulation of what came to be known as the “verification principle,” according to which a sentence is meaningful only if it is either tautologous or in principle verifiable on the basis of sense experience.

Later developments in epistemology served to make some empiricist ideas about knowledge and justification more attractive. One of the traditional problems faced by more radical forms of empiricism was that they seemed to provide too slender a foundation upon which to justify what humans think they know. If sensations can occur in the absence of physical objects, for example, and if what one knows immediately is only the character of one’s own sensations, how can one legitimately infer knowledge of anything else? Hume argued that the existence of a sensation is not a reliable indicator of anything other than itself. In contrast, adherents of a contemporary school of epistemology known as “externalism” have argued that sensations (and other mental states) can play a role in justifying what humans think they know, even though the vast majority of humans are unaware of what that role is. The crude idea behind one form of externalism, “reliablism,” is that a belief is justified when it is produced through a reliable process—i.e., a process that reliably produces true beliefs. Humans may be evolutionarily conditioned to respond to certain kinds of sensory stimuli with a host of generally true, hence justified, beliefs about their environment. Thus, within the framework of externalist epistemology, empiricism might not lead so easily to skepticism.